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draft-papadopoulos-roll-dis-mods-use-cases.txt
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ROLL G. Papadopoulos
Internet-Draft IMT Atlantique
Intended status: Standards Track March 9, 2020
Expires: September 10, 2020
Use cases for DIS Modifications
draft-papadopoulos-roll-dis-mods-use-cases-00
Abstract
This document presents some of the use-cases which call for DIS flags
and options modifications.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Papadopoulos Expires September 10, 2020 [Page 1]
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Applications . . . . . . . . . . . . . . . . . . . . . . . . 2
3.1. A Leaf Node Joining a DAG . . . . . . . . . . . . . . . . 2
3.2. Identifying A Defunct DAG . . . . . . . . . . . . . . . . 3
3.3. Adjacencies probing with RPL . . . . . . . . . . . . . . 5
3.3.1. Deliberations . . . . . . . . . . . . . . . . . . . . 6
4. Informative References . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Applications
This section details some use cases that require DIS modifications
compared to the behaviour currently defined in [RFC6550]. The first
use case is thatof a new leaf node joining an established DAG in an
energy efficient manner. The second use case describes why node
might want to use DIS to identify defunct DAGs for which it still
maintains state. The third use case describes the need for adjacency
probing and how DIS can used for that.
3.1. A Leaf Node Joining a DAG
This use case is typically of a smart meter being replaced in the
field, while a RPL network is operating and stable. The new smart
meter must join the network quickly, without draining the energy of
the surrounding nodes, be they battery-operated RPL routers or leaf
nodes. In this use case, the issues with the current RPL
specification are
o Just waiting for a gratuitous DIO may take a long time if the
Trickle timers have relaxed to the steady state. A technician who
has just installed the new meter needs to positively assess that
the meter has joined the network before it leaves the premise. It
is not economically viable to ask the technician to standby the
meter until a gratuitous DIO has arrived, which may take hours.
o If the meter sends a DIS, it needs to do so using multicast,
because it has no knowledge of its surroundings. Sending a
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multicast DIS is considered an inconsistency by the nearby RPL
routers. They will reset their Trickle timer to the shortest
period. This will trigger sending a stream of DIOs until the
Trickle timers relax again. The DIOs will be sent in multicast,
which will trigger energy expenditure at nearby nodes, which had
no need for the DIOs.
A proposed solution could be the following. A new leaf node that
joins an established LLN runs an iterative algorithm in which it
requests (using multicast DIS) DIOs from routers belonging to the
desired DAG.
The DIS message has the "No Inconsistency" flag set to prevent
resetting of Trickle timer in responding routers, thereby keeping the
aggregated number of transmissions low. It also has the "DIO Type"
flag set to make responding routers send unicast DIOs back, thereby
not triggering full reception in nearby nodes that have state-of-the-
art radio receivers with hardware-based address filtering.
The DIS message can include a Response Spreading option prescribing a
suitable spreading interval based on the expected density of nearby
routers and on the expected Layer 2 technology.
The DIS will likely include a Metric Container listing the routing
constraints that the responding routers must satisfy in order to be
allowed to respond [RFC6551].
At each iteration, the node multicasts such a DIS and waits for
forthcoming DIOs. After a time equal to the spreading interval, the
node considers the current iteration to be unsuccessful. The node
consequently relaxes the routing constraints somewhat and proceeds to
the next iteration.
The cycle repeats until the node receives one or more DIOs or until
it has relaxed the constraints to the lowest acceptable values.
This algorithm has been proven in the field to be extremely energy-
efficient, especially when routers have a wide communication range.
3.2. Identifying A Defunct DAG
A RPL node may remove a neighbor from its parent set for a DAG for a
number of reasons:
o The neighbor is no longer reachable, as determined using a
mechanism such as Neighbor Unreachanility Detection (NUD)
[RFC4861], Bidirectional Forwarding Detection (BFD) [RFC5881] or
L2 triggers [RFC5184]; or
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o The neighbor advertises an infinite rank in the DAG; or
o Keeping the neighbor as a parent would required the node to
increase its rank beyond L + DAGMaxRankIncrease, where L is the
minimum rank the node has had in this DAG; or
o The neighbor advertises membership in a different DAG within the
same RPL Instance, where a different DAG is recognised by a
different DODAGID or a different DODAGVersionNumber.
Even if the conditions listed above exist, a RPL node may fail to
remove a neighbor from its parent set because:
o The node may fail to receive the neighbor's DIOs advertising an
increased rank or the neighbor's membership in a different DAG;
o The node may not check, and hence may not detect, the neighbor's
unreachability for a long time. For example, the node may not
have any data to send to this neighbor and hence may not encounter
any event (such as failure to send data to this neighbor) that
would trigger a check for the neighbor's reachability.
In such cases, a node would continue to consider itself attached to a
DAG even if all its parents in the DAG are unreachable or have moved
to different DAGs. Such a DAG can be characterized as being defunct
from the node's perspective. If the node maintains state about a
large number of defunct DAGs, such state may prevent a considerable
portion of the total memory in the node from being available for more
useful purposes.
To alleviate the problem described above, a RPL node may invoke the
following procedure to identify a defunct DAG and delete the state it
maintains for this DAG. Note that, given the proactive nature of RPL
protocol, the lack of data traffic using a DAG can not be considered
a reliable indication of the DAG's defunction. Further, the Trickle
timer based control of DIO transmissions means the possibility of an
indefinite delay in the receipt of a new DIO from a functional DAG
parent. Hence, the mechanism described here is based on the use of a
DIS message to solicit DIOs about a DAG suspected of defunction.
Further, a multicast DIS is used so as to avoid the need to query
each parent individually and also to discover other neighbor routers
that may serve as the node's new parents in the DAG.
When a RPL node has not received a DIO from any of its parents in a
DAG for more than a locally configured time duration:
o The node generates a multicast DIS message with:
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* the "No Inconsistency" flag set so that the responding routers
do not reset their Trickle timers.
* the "DIO Type" flag not set so that the responding routers send
multicast DIOs and other nodes in the vicinity do not need to
invoke this procedure.
* a Solicited Information option to identify the DAG in question.
This option must have the I and D flags set and the
RPLInstanceID/DODAGID fields must be set to values identifying
the DAG. The V flag inside the Solicited Information option
should not be set so as to allow the neighbors to send DIOs
advertising the latest version of the DAG.
* a Response Spreading option specifying a suitable time interval
over which the DIO responses may arrive.
o After sending the DIS, the node waits for the duration specified
inside the Response Spreading option to receive the DIOs generated
by its neighbors. At the conclusion of the wait duration:
* If the node has received one or more DIOs advertising newer
version(s) of the DAG, it joins the latest version of the DAG,
selects a new parent set among the neighbors advertising the
latest DAG version and marks the DAG status as functional.
* Otherwise, if the node has not received a DIO advertising the
current version of the DAG from a neighbor in the parent set,
it removes that neighbor from the parent set. As a result, if
the node has no parent left in the DAG, it marks the DAG as
defunct and schedule the deletion of the state it has
maintained for the DAG after a locally configured "hold"
duration. (This is because, as per RPL specification, when a
node no longer has any parents left in a DAG, it is still
required to remember the DAG's identity (RPLInstanceID,
DODAGID, DODAGVersionNumber), the lowest rank (L) it has had in
this DAG and the DAGMaxRankIncrease value for the DAG for a
certain time interval to ensure that the node does not join an
earlier version of the DAG and does not rejoin the current
version of the DAG at a rank higher than L +
DAGMaxRankIncrease.)
3.3. Adjacencies probing with RPL
RPL avoids periodic hello messaging as compared to other distance
vector protocols. It uses trickle timer based mechanism to update
configuration parameters. This significantly reduces the RPL control
overhead. One of the fallout of this design choice is that, in the
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absence of regular traffic, the adjacencies could not be tested and
repaired if broken.
RPL provides a mechanism in the form of unicast DIS to query a
particular node for its DIO. A node receiving a unicast DIS MUST
respond with a unicast DIO with Configuration Option. This mechanism
could as well be made use of for probing adjacencies and certain
implementations such as Contiki uses this. The periodicity of the
probing is implementation dependent, but the node is expected to
invoke probing only when
o There is no data traffic based on which the links could be tested.
o There is no L2 feedback. In some case, L2 might provide periodic
beacons at link layer and the absence of beacons could be used for
link tests.
3.3.1. Deliberations
o Should the probing scheme be standardized? In some cases using
multicast based probing may prove advantageous.
o In some cases using multicast based probing may prove
advantageous. Currently RPL does not have multicast based
probing. Multicast DIS/DIO may not be suitable for probing
because it could possibly lead to change of states.
4. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC5184] Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
(L3)-Driven Fast Handover", RFC 5184,
DOI 10.17487/RFC5184, May 2008,
<https://www.rfc-editor.org/info/rfc5184>.
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[RFC5881] Katz, D. and D. Ward, "Bidirectional Forwarding Detection
(BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
DOI 10.17487/RFC5881, June 2010,
<https://www.rfc-editor.org/info/rfc5881>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6551] Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
and D. Barthel, "Routing Metrics Used for Path Calculation
in Low-Power and Lossy Networks", RFC 6551,
DOI 10.17487/RFC6551, March 2012,
<https://www.rfc-editor.org/info/rfc6551>.
Author's Address
Georgios Z. Papadopoulos
IMT Atlantique
Office B00 - 102A
2 Rue de la Chataigneraie
Cesson-Sevigne - Rennes 35510
FRANCE
Phone: +33 299 12 70 04
Email: [email protected]
Papadopoulos Expires September 10, 2020 [Page 7]